Nystagmus is an involuntary jerking or bouncing of the eyeball that occurs when there is a disturbance of the vestibular (inner ear) system or the oculo-motor control of the eye. Horizontal gaze nystagmus (HGN) refers to a lateral or horizontal jerking when the eye gazes to the side. In the impaired driving context, alcohol consumption or consumption of certain other central nervous system depressants, inhalants or phencyclidine, hinders the ability of the brain to correctly control eye muscles causing the jerk or bounce associated with HGN. As the degree of impairment becomes greater, the jerking or bouncing of the nystagmus becomes more pronounced. This is assessed in the horizontal gaze nystagmus test.
The horizontal gaze nystagmus test is one of three field sobriety tests that comprise the standardized field sobriety test (SFST) battery (the other two tests are the walk-and-turn test and the one-leg-stand test) of the National Highway Traffic Safety Administration (NHTSA). Scientific evidence has shown the horizontal gaze nystagmus test as a reliable roadside measure of a person's impairment due to alcohol or certain other drugs.
Nystagmus describes the “bouncing” eye motion displayed in two ways: (1) pendular nystagmus, where the eye oscillates equally in two directions, and (2) jerk nystagmus, where the eye moves slowly away from a fixation point and then is rapidly corrected through a saccadic (fast) movement. HGN is a type of jerk nystagmus with the saccadic movement toward the direction of the gaze.
An eye normally moves smoothly, whereas an eye with jerk nystagmus moves like a marble rolling across sandpaper. Most types of nystagmus, including HGN, are involuntary motions, meaning the person exhibits the nystagmus without control. The subject exhibiting nystagmus is unaware that it is happening because the bouncing of the eye does not affect the subject's vision.
Law enforcement officers have used field sobriety tests (FST) to detect impairment and to develop probable cause to arrest. Most FSTs test coordination, Balance, and dexterity. The HGN test tracks the movement or coordination and balance of the eye.
The HGN test requires an object for a subject to follow with their eye, such as a pen or the tip of a penlight. The officer places the object approximately twelve to fifteen inches from the subject's face and slightly higher than eye level and instructs the subject to follow the object with the eyes only with the head held still.
The officer checks for the lack of smooth pursuit by eye by moving the object slowly from the center of the subject's face towards the left ear. The left eye should smoothly follow the object, but if the eye exhibits nystagmus, the officer notes the clue. The officer then checks the right eye for the same.
Starting again from the center of the suspect's face, the officer moves the object toward the left ear, bringing the eye as far over as possible and holds the object there for four seconds. The officer notes if there is a distinct and sustained nystagmus at this point. The officer holds the object at maximum deviation for at least four seconds to ensure that quick movement of the object did not cause the nystagmus. The right eye is then checked using the same procedure. The procedure is commonly referred to as “end-point” nystagmus.
The test further requires the officer to move the object at a speed that would take about four seconds for the object to reach the edge of the suspect's left shoulder. The officer notes if the point or angle at which the eye begins to display nystagmus is before the object reaches forty-five degrees from the center of the suspect's face. The officer then moves the object towards the suspect's right shoulder.
Despite the strong correlation between alcohol consumption and HGN, some trial courts still do not admit the results of the HGN test into evidence. Additionally, even if admitted, the officer's administration of the test is often challenged. Therefore, there is a need for an HGN testing device capable of administering and recording the results of an HGN test more consistently and effectively.